Plan B reduces the risk of pregnancy by roughly 85% when taken within 72 hours of unprotected sex, but its effectiveness drops significantly with every day you wait. Taken within the first 24 hours, it works best. By days four and five, pregnancy rates climb noticeably. How well it works for you also depends on your body weight and where you are in your menstrual cycle.
How Plan B Prevents Pregnancy
Plan B contains a synthetic hormone that works by stopping or delaying ovulation. If your body hasn’t released an egg yet, the hormone essentially puts that process on pause so there’s no egg available to be fertilized. This is the primary way it works.
If ovulation has already happened, Plan B is far less effective. It does not prevent a fertilized egg from implanting, and it cannot end a pregnancy that has already begun. This is why timing matters so much: the pill is most useful when taken before ovulation occurs, giving it time to block the process entirely.
How Timing Changes Your Odds
The single biggest factor in how well Plan B works is how quickly you take it. The general guidance is to take it as soon as possible, ideally within the first 24 hours. Every additional day reduces its effectiveness.
Five studies reviewed by the CDC found that pregnancy risk stays low through the first three days (72 hours) but begins rising after that. A meta-analysis of the data showed pregnancy rates remained low when the pill was taken within four days but increased at four to five days. Technically, the CDC notes Plan B can be taken up to five days (120 hours) after unprotected sex, but waiting that long means you’re relying on significantly weaker protection.
The bottom line: if you’re going to take Plan B, take it now rather than later. Waiting even 12 extra hours can make a measurable difference.
Weight Affects How Well It Works
Research has shown that Plan B becomes less reliable as body weight increases. A study found that the pill was less likely to work for women with a BMI above 26, which roughly corresponds to about 155 pounds for someone who is 5’4″. Of all emergency contraception options, levonorgestrel (the active ingredient in Plan B) is the most likely to fail for women with a higher BMI.
If your weight puts you above that threshold, you have better alternatives. A prescription option called ella works more effectively than Plan B regardless of when you take it within its five-day window, and it holds up better across a wider range of body weights. The copper IUD, which a provider can place up to five days after unprotected sex, is the most effective emergency contraception available and works regardless of weight.
How Plan B Compares to Other Options
Plan B is the most accessible emergency contraceptive. It’s available over the counter without a prescription or age restriction. But it’s not the most effective option.
- Plan B (levonorgestrel): Available over the counter. Effective up to three days, with declining efficacy through five days. Less reliable at higher body weights.
- Ella (ulipristal acetate): Requires a prescription. More effective than Plan B at every time point within its five-day window. Better performance across different body weights.
- Copper IUD: Requires a clinic visit for insertion. The most effective emergency contraceptive available, reducing pregnancy risk by over 99%. Works regardless of weight. Can also serve as long-term birth control for up to 10 years afterward.
If you have time and access to a provider, ella or the copper IUD will give you stronger protection, especially if it’s been more than 24 hours or if you weigh more than 155 pounds.
What to Expect After Taking It
Plan B can cause nausea, fatigue, headache, and breast tenderness. These side effects are typically mild and resolve within a day or two. If you vomit within three hours of taking the pill, the CDC recommends taking another dose as soon as possible, since your body may not have absorbed enough of the hormone.
Your next period may come earlier or later than expected. A shift of several days in either direction is normal. If your period doesn’t arrive within three weeks, take a pregnancy test. Plan B is not 100% effective, and a delayed period can be an early sign of pregnancy.
After taking Plan B, you can start or resume a regular birth control method right away. But you’ll need to use condoms or abstain from sex for seven days, since hormonal birth control takes about a week to become fully effective after starting.
Medications That Can Interfere
Certain medications speed up how quickly your liver processes hormones, which can make Plan B less effective. Anti-seizure medications like phenytoin, carbamazepine, and topiramate are known to reduce hormonal contraception’s effectiveness. The antibiotic rifampin, used primarily for tuberculosis, has the same effect. The herbal supplement St. John’s Wort, commonly taken for mood, also speeds up hormone metabolism.
If you take any of these, the copper IUD is a better emergency option since it doesn’t rely on hormones at all. Let your provider know what medications you’re on so they can recommend the most reliable choice for your situation.